The present invention relates to anti-lock control for preventing a wheel lock when braking a running vehicle.
Vehicle anti-lock control systems are designed to ensure steering and running stability during braking and to reduce braking distance. In conventional anti-lock control arrangements, a control mode of brake hydraulic pressure is determined in accordance with an electric signal representative of a wheel speed detected by a wheel speed sensor. Based upon the signal, a control unit having a microcomputer controls a hold valve, in the form of a normally-open type solenoid valve, and a decay valve, in the form of a normally-closed type solenoid valve, to increase, decrease or hold the brake hydraulic pressure.
In a conventional anti-lock control arrangement, a reference speed for deciding when to decrease pressure is set or determined in accordance with a wheel speed (hereinafter referred to as "system speed Vs") to be controlled in each brake control system. There is also set an estimated vehicle speed Vv having a predetermined follow-up limit of the acceleration and deceleration with respect to the highest wheel speed among four wheel speeds. Further, there is set a threshold speed which tracks the estimated vehicle speed Vv by a predetermined value below the estimated vehicle speed. Based on the comparison of the system speed Vs with the reference speed or the threshold speed and also on the detected high peak point and low peak point of the system speed Vs, a pressure decrease status, a pressure hold status and a pressure increase status are set. Then, a plurality of hydraulic pressure control valves (solenoid valves) are turned on and off in accordance with a predetermined control mode set for each of these statuses, thereby effecting the pressure decrease, pressure hold and pressure increase.
However, in this conventional control method, the control mode remains fixed or unchanged until the status is changed, even when the wheel speed is abruptly changed due to variations in the road surface condition. Thus, conventional methods fail to react in a timely manner which is particularly disadvantageous because the wheels may lock during abrupt braking. Another disadvantage is that since the pressure increase is effected at the time when the wheel speed is recovered to a speed near the vehicle speed, the wheel speed is repeatedly accelerated and decelerated in a speed range having a certain slip rate with respect to the vehicle speed. As a result, the brake hydraulic pressure is increased and decreased causing a vibration of the vehicle body.